This invention relates to an improved method for separating and recycling hot fines, and is especially useful in recycling hot fines produced in the hot briquetting of reduced iron.
Hot briquetting of reduced iron consists of feeding the output material of a direct reduced iron (DRI) process, for example, from a direct reduction shaft furnace, into a press in which the material is formed into uniform, pillow-shaped briquettes. The material is substantially metallized with an iron content exceeding 85%. The material temperature at the egress of the furnace depends on the type of furnace used, but is greater than 600xc2x0 C. in all cases. The material is not cooled before the briquetting operation. The briquettes usually are formed into sheets of connected briquettes. A breaking operation follows the briquetting press so that briquettes abutting and connected in the press are separated at the egress of the briquette process. xe2x80x9cFinesxe2x80x9d are particles of DRI that consist of shards of small (less than 12 mm) irregular shapes and other particulates that are created in the breaking operation. These fines must be removed from the egress material but are nonetheless valuable DRI material. Therefore, fines must be removed and recycled in any economically feasible method.
The current State of the Art for recovering fines is to pass hot direct reduced iron, DRI briquettes, shards and fines over a vibrating screen through which the small particles fall, then recycle the fines to a briquette feed screw via a bucket elevator. This method involves mechanical equipment and concomitant maintenance problems. For example, when the vibrating screen becomes hot it loses surface hardness, and therefore wears out quickly. Replacement of the screen is expensive and requires process interruption. The hot bucket elevators require excessive maintenance. These bucket elevators have many moving parts which are subject to undesired thermal expansion and creep. Ensuring that the system is hermetic and that only inert gases contact the hot DRI is also a problem with known fines separation and recycling systems. Hot DRI fines are subject to reoxidation and burning, including catastrophic burning or explosion upon exposure to oxygen.
Applicants are aware of the following prior art:
Biscofsheim et al., U.S. Pat. No. 3,630,353, teaches a method for removing fines or dust from broken sinter cake before delivery to a blast furnace. Biscofsheim works by blowing air upward through a perforated bottom wall of a rotating cooler and through the broken sinter thereon. The fines are carried by the air to an exhaust hood and then to a precipitator where the fines are collected and then reintroduced into the sintering machine. Air pressure and air velocity are changed according to the thickness of sinter material and the size of the fines to be collected.
Rohaus, U.S. Pat. No. 3,116,996, teaches a method for handling fines which remain after a briquetting operation without the use of a cyclone or a screen. Rohaus handles fines by blowing the briquettes with air while in a cooling shaft and reintroducing the fines into a kiln. Rohaus does not mention the reduced or oxidized status of the briquettes or fines, but seems more concerned with moisture content. The method utilizes the air, heated by the briquettes, as secondary combustion air within the kiln. The recycled fines are passed only through a final portion of the kiln since the fines are already substantially moisture free. Howell, U.S. Pat. No. 4,249,906, teaches a process for recovering and using fines of flux, such as limestone, used in metallurgical processes. Howell collects airborne fines with a baghouse or a similar piece of equipment and the fines are then transported to a first processing zone. The method of transportation is called xe2x80x98dense phase collectionxe2x80x99, a method in which particulate matter is carried by a gas in a manner resembling a fluidized bed. The material is formed into briquettes. The briquettes, fines and larger pieces of flux are temporarily stored. Upon removal from storage the material is abrasively deflashed, then transported to a vibrating screen classifier. Appropriately sized pieces are delivered for processing. Other sizes, both too large and too small are recycled to the briquetting machine.
Metz, U.S. Pat. No. 4,274,863, teaches a method oftreating flue gas. Metz describes a single gas treatment station used to clean varying gases from the steel mill before discharge to the environment. The flue gas containing fines and other contaminants is introduced into a sintering plant. The flue gas is introduced with air and incorporated into the combustion reaction inside the sintering plant. Many of the pollutants in the flue gas are oxidized, and the dust in the flue gas is affixed to the sintering material. Pollutants may be selectively added to the sintering process in the location where materials are typically decomposed or removed. For instance, those gas streams containing large amounts of sulfuric components may be introduced into the upstream zone of the sintering bed. Metz also provides that crude dust may be removed before the gas and dust mixture enters the sintering plant.
Coyne, Jr., U.S. Pat. No. 4,917,723, teaches a method for capturing nonferrous materials used in metallurgical processes. The process begins by collecting plant waste including fines, sludge, dust, etc. The captured dust and fines are fed to a briquette press and then to a smelter. The process is repeated so that nonferrous constituent material is repeatedly captured, briquetted, smelted, volatilized, cooled, then captured again. This process continues until the dust has a sufficient percentage of nonferrous material to warrant removal and recovery. The goal of the process is to recycle iron containing material while recovering nonferrous material. This patent applies to direct reduced iron though it is not limited to DRI.
Batterham et al., U.S. Pat. No. 5,364,446, teaches a process for heating solid non-reducible particulate material by carrying it in a hot gas. This process may be used in the production of Portland type cement or may be used to heat carbonate materials for a metallurgical process. The detailed description in the patent uses an off gas to heat a carbonate in preparation for calcination. Problems with stickiness from molten or sticky particles in the off gas are solved by cooling particles slightly or establishing flow patterns to minimize the interaction between particles to be carried by the flue gas and particles already present in the flue gas. The goal of this invention is to heat carbonate material with flue gas with minimal interaction between the carbonate material and the particulate matter in the flue gas.
The present invention differs substantively from the teachings of the above references. The invented process requires that the briquette temperature at the egress of the briquette operation is not substantially lower than the temperature egress of the DRI furnace. Entraining DRI fines at more than 600xc2x0 C. in air is inoperable since hot DRI readily, possibly explosively, reacts with oxygen. Note that the term xe2x80x9cfinesxe2x80x9d in the present invention is defined as particulate materials, the size of which particles range from less than 0.1 mm dust to 12 mm shards of DRI material.
The invention is a method and apparatus for separating and recycling hot DRI fines produced in the hot briquetting of reduced iron. The method pneumatically separates fines from briquettes upon breaking of the briquettes, and pneumatically transports the fines to the material feed inlet of the briquetting machine. This method eliminates the need for a hot screen and for a bucket elevator, both of which require unacceptably high maintenance expense. The invented method utilizes a hermetic system to avoid problems with re-oxidation, such as combustion, or even explosion. It also eliminates the need for a separate dust collection system.
The principal object of this invention is to provide a mechanically simplified method of separating and recycling fines and shards that occur in the breaker chamber of a briquetting machine.
It is another object of provide a method of recovering waste fines from a briquetting machine when such machine is used for briquetting hot, substantially metallized, iron-bearing materials.
Another object of the invention is to provide an improved method and apparatus for recovering and recycling hot direct reduced iron fines which is reliable and requires low maintenance.